Our overall objective is to observe and examine the relationship between the structure (chemical and stereochemical) of the snake venom neurotoxins and their functions. These are small highly basic single chain proteins with intra chain disulphide bridges essential for toxicity. The dominant groups of these toxins act to produce a post synaptic neuromuscular non-depolarizing block associated with binding to the membrane receptor protein. Our immediate aim is to complete the structure analyses of the erabutoxins a, b, and c (venoms from the sea snake laticauda semi- fasciata) and laticotoxin (a venom from the sea snake laticauda laticaudata). We are studying a three-dimensional electron density Fourier map of erabutoxin b at 3.5 A resolution. The multiple isomorphous replacement technique (five heavy-atom derivatives) was employed in calculating the phases. Once the structure of erabutoxin b has been established, an interpretable map of the isomorphous erabutoxin a should readily be generated from the erabutoxin b phases; a rigid body search procedure (based on the b structure) will be used for structure determination in the non-isomorphous erabutoxin c and laticotoxin crystals.